This indicator shows the observed trends in greenhouse gas concentrations. Greenhouse gases differ in the way they affect the climate system. In order to sum the effects of the individual greenhouse gasses and other forcing agents in the atmosphere, the so-called ‘greenhouse gas equivalent concentration’ has been defined. This is the concentration of CO 2 that would cause the same amount of radiative forcing as a mixture of CO 2 and other forcing agents (greenhouse gases and aerosols). The forcing is expressed here in CO 2- equivalent concentrations, rather than watts per square meter (W/m 2 ), because they are more easily understood by the general public. For a more detailed assessment of radiative forcing expressed in W/m 2 , the reader is referred to IPCC (2013).
Global average annual concentrations are presented here. Although greenhouse gases are mainly emitted in the northern hemisphere, the use of global average values is considered justified, because the atmospheric lifetime of most greenhouse gases is long compared to the timescales of global atmospheric mixing. This leads to a rather uniform mixture around the globe. The exceptions are ozone and aerosols. However, as described earlier, these gases are less relevant, over the long term.

Assessment versions

Published (reviewed and quality assured)

No published assessments

Rationale

Justification for indicator selection

Greenhouse gases can affect the radiation balance on Earth and are, therefore, crucial for the global climate. Without these gases, the global average temperature would be about 32 oC lower than it is now (i.e. -18 oC instead of the current +14 oC global mean temperature average).

The various greenhouse gases affect the climate system in different ways. In order to sum their effects on the atmosphere, the so-called greenhouse gas equivalent concentration has been defined. This is the concentration of CO2 that would cause the same amount of radiative forcing as the mixture of CO2 and other greenhouse gasess over a 100-year time horizon. CO2-equivalent concentrations, rather than radiative forcings, are presented here, because they are more easily understood by the general public. For an overview on radiative forcing, the reader is referred to IPCC (2013). A summary of radiative forcing of different greenhouse gases can be found here.

There are, in general, three ways the greenhouse gas equivalent concentration can be aggregated, all of which are presented here. First, an approach often used is to group together the six main human-made greenhouse gases under the Kyoto Protocol (i.e. CO2, CH4, N2O, HFCs, PFCs and SF6). A second approach is to group all long-living greenhouse gases (i.e. Kyoto Protocol gases plus the Montreal Protocol gases (i.e. CFCs, HCFCs & CH3CCl3)). Velders et al (2007) have shown that reducing the concentration of these Montreal gases has a considerable beneficial effect for the climate. Finally, in a third approach, ozone, water vapour and aerosols can also be included to show the net effect of all greenhouse gases on the Earth’s radiation balance, and as such, the climate system. Unfortunately the data record of this third approach is shorter due to the lack of long-term data series for ozone. Furthermore, the forcing of many of the aerosol species is highly uncertain (e.g. they have also changed between IPCC (2007) and IPCC (2013). See table below).

Considering all greenhouse gases together can provide a lower concentration level compared to the other two approaches due to the net cooling effect of aerosols. For the current period (e.g. 2012), the three approaches lead to quite different concentration levels (see assessment), but for the long-term the three approaches could converge as a result of the decrease in Montreal Protocol gases that is starting to occur (Montzka et al, 2011). Global sulphur and aerosols emissions are also likely to be reduced due to non-climate related policies.

UNFCCC (2009)
UNFCCC (2009). Report of the Conference of the Parties on its fifteenth session, held in Copenhagen from 7 to 19 December 2009; Part Two: Decisions adopted by the Conference of the Parties

Indicator definition

This indicator shows the observed trends in greenhouse gas concentrations. Greenhouse gases differ in the way they affect the climate system. In order to sum the effects of the individual greenhouse gasses and other forcing agents in the atmosphere, the so-called ‘greenhouse gas equivalent concentration’ has been defined. This is the concentration of CO2 that would cause the same amount of radiative forcing as a mixture of CO2 and other forcing agents (greenhouse gases and aerosols). The forcing is expressed here in CO2-equivalent concentrations, rather than watts per square meter (W/m2), because they are more easily understood by the general public. For a more detailed assessment of radiative forcing expressed in W/m2, the reader is referred to IPCC (2013).

Global average annual concentrations are presented here. Although greenhouse gases are mainly emitted in the northern hemisphere, the use of global average values is considered justified, because the atmospheric lifetime of most greenhouse gases is long compared to the timescales of global atmospheric mixing. This leads to a rather uniform mixture around the globe. The exceptions are ozone and aerosols. However, as described earlier, these gases are less relevant, over the long term.

Units

Atmospheric concentration is measured in parts per million CO2-equivalents (ppm CO2 eq.).

Policy context and targets

Context description

The overall objective of the United Nations Framework Convention on Climate Change (UNFCCC), is ‘to stabilise atmospheric greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interference with the climate system’ (UNFCCC, 1993). Both at global level (UNFCCC, 2009) and EU levels (October 2008 Environment Council conclusions), this ‘dangerous anthropogenic interference’ has been recognised in an ambition to keep the long-term global average temperature rise below 2 °C, compared to pre-industrial levels. In December 2015, the Paris Agreement strengthened this by stating its objective as ‘holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels’(UNFCC, 2015).

CO2 equivalent concentration levels can be defined that correspond to these and other temperature increases. Note that these temperature increases represent a specific long-term equilibrium warming. The equilibrium climate sensitivity is defined as the change in global mean temperature, T2x, that results when the climate system, or a climate model, attains a new equilibrium with the forcing change, F2x, resulting from a doubling of the atmospheric CO2 concentration (IPCC, 2013).

Studies have been done to assess the probability of achieving this objective. These include both the 1.5 °C and 2 °C targets (above pre-industrial levels) in relation to various stabilisation levels of greenhouse gases in the atmosphere (Meinshausen et al., 2009, 2011; Van Vuuren et al., 2011; IPCC, 2014), which would result in the main objective of the COP21 Paris agreement being achieved (see also Table 1).

From estimates on climate sensitivity, it can be derived that the concentration of all greenhouse gases in the atmosphere – including ozone, water vapour and aerosols – would need to remain below approximately 430 ppm CO2 equivalent to achieve a 50 % probability of keeping the increase in global mean temperature below 1.5 °C. This concentration that is lower than the one currently observed. The atmospheric concentration level of all greenhouse gases that would be consistent with a maximum temperature increase of 2 °C is between 480 ppm and 650 ppm CO2 equivalent. Respectively, these figures give a 67 % and <10 % probability of staying below these temperature thresholds, see Table 1).

Targets

In 1992, countries adopted the United Nations Framework Convention on Climate Change (UNFCCC) to cooperatively consider options for limiting average global temperature increases and the resulting climate change.[1] Continuous discussions under the UNFCCC led to the adoption, in 1997, of the Kyoto Protocol [2], which legally binds developed countries to achieve greenhouse gas emissions reduction targets.

In 2010, the international community agreed on the need to reduce emissions in order to prevent global temperature increases from exceeding 2 °C compared to pre-industrial levels[3] (no more than 1 °C above today's level). Scenarios consistent with this target show global emissions reductions of 40 % to 70% by 2050, compared to 2010 levels (i.e. 40 % to 60 % for scenarios with negative emissions, and 60 % to 70% for scenarios without negative emissions).[4]

More than 90 countries agreed to take on mitigation commitments until 2020, including the major developed and developing nations. The European Union (EU) and a small additional number of developed countries made their commitments under the Doha Amendment to the Kyoto Protocol for a second commitment period running from 2013 to 2020.[5] To secure the chance to stay below 2 °C, the international community has decided to work towards an international climate agreement for the period after 2020, which should be applicable to all. Such a new global agreement was concluded at the COP21 meeting in Paris, in November 2015. The sum of the proposed emissions reductions for all countries would still be insufficient to meet the target (UN Gap analysis).

Related policy documents

Council Decision (2002/358/EC) of 25 April 2002 concerning the approval, on behalf of the European Community, of the Kyoto Protocol to the United Nations Framework Convention on Climate Change and the joint fulfilment of commitments thereunder.

Decision No 280/2004/EC of the European Parliament and of the Council of 11 February 2004 concerning a mechanism for monitoring Community greenhouse gas emissions and for implementing the Kyoto Protocol

The Paris Agreement. Report of the Conference of the Parties on its twenty-first session, held in Paris from 30 November to 11 December 2015.

Key policy question

How are atmospheric concentrations of all greenhouse gases developing over time?

Methodology

Methodology for indicator calculation

The trends in global average concentrations of atmospheric CO2 are taken directly from NOAA (NOAA, 2015, see also www.esrl.noaa.gov/gmd/ccgg/trends/global.html). Global average concentration values for the other gases are mainly based on AGAGE figures (2014) (see agage.eas.gatech.edu/index.htm). These global figures were derived by averaging the data from four observatory stations across the world, equally distributed over the northern and southern hemispheres.

Radiative forcings are calculated using an approximate equation according to IPCC (2013), based on the observed atmospheric concentrations and using radiative efficiencies for CO2, CH4, N2O, ozone (both stratospheric and tropospheric) and vapour. IPCC (2013) estimates were also used for the radiative forcing of aerosols between 1970 and 2013.

The equations used to compute the contribution of individual gases are presented below:

Trace gas

Parameterisation, radiative forcing, change in forcing (F) (Wm-2)

Constants (IPCC, 2013, Butler, 2009)

CO2

change in F = alpha ln (C/C0)

C and C0 are the current and pre-industrial concentrations (ppm) of CO2, respectively

M and M0 are the current and pre-industrial concentrations of CH4, respectively; N and N0 are the current and pre-industrial concentrations of N2O, respectively.

alpha = 0.12

HFC, PFC & SF6

change in F = alpha (X-X0)

X and X0 are the current and pre-industrial concentrations (ppb) of gas X, respectively.

Values for alpha depend on molecule (see below), taken from WMO, 2002.

A similar approach was applied for the Montreal Protocol gases (i.e. CFCs and HCFCs):

CFCs and HCFCs

change in F = alpha (X-X0)

X and X0 are the current and pre-industrial concentrations (ppb) of gas X, respectively.

Values for alpha depending on molecule (see below), taken from WMO, 2002.

Overview of used alpha values for chlorine Kyoto and Montreal Gases (also see CDIAC):

Kyoto gases

Alpha values

Montreal gases

Alpha values

HFC-23

0.16

CFC-11

0.25

HFC-134a

0.159

CFC-12

0.32

CF4

0.116

CFC-13

0.25

C2F6

0.26

CFC-113

0.3

SF6

0.52

CFC-114

0.31

HFC-23

0.16

CFC-115

0.18

HCFC-22

0.2

HCFC-141

0.14

HCFC-142

0.163

CCl

0.13

CH4CC

0.01

CH4CCl3

0.06

CH3Br

0.05

In calculating the radiative forcing (and accompanying concentration levels) of the Montreal Protocol gases, the effect of ozone depleting substances on the stratospheric ozone layer was also considered. A factor of -0.154 for correcting the forcing of Montreal Protocol gases, based on IPCC (2013) (= -0.05 W/m2 or 10 ppm in 2014), was used.

To quantify the total concentration of all greenhouse gases, the forcing of tropospheric ozone, water vapour in the atmosphere, cloud interaction, changes in albedo (e.g. due to black carbon) and the direct effects of multiple aerosols were added. Due to uncertainties in the measurements, and the large inter-annual and seasonal variation, forcing for tropospheric ozone, water vapour and land-use changewas kept constant over the years at a respective 0.4, 0.07 (IPCC, 2013, p. 696) and 0.15 W/m2 (IPCC, 2013, p. 698).

For aerosols, we used figures (i) based on recent the IPCC report. For example, the forcing of black carbon (BC) from fossil fuel and biofuel use has increased as a result of increased emissions, especially in east and southeast Asia; (ii) changed the methodology for the period from 1970 to 2014. In this new methodology radiative forcing figures for for sulphate, black carbon, organic carbon, nitrate and other forcers (= sum of mineral dust, cloud interaction and contrails), based on IPCC (2013, Fig. 8.17, pg. 698), were defined. These figures are provided in the table below. Then the radiative forcing for these five groups back to 1970 was calculated. For sulphate, historical emissions data from Klimont et al. (2013) and IPCC (2013, Figure 8.8, p. 683) were used. For black carbon, organic carbon and nitrate, we used the historical radiative forcing figures as given by the IPCC (Figure 8.8, p. 683). Note that here the forcing of biomass burning was kept to zero, as it is balanced by a positive forcing due to black carbon and negative forcing from organic carbon (see also IPCC, 2013, Table 8.4).

Calculated radiative forcing for multiple aerosols for some years in the past (all in W/m2) is presented below.

Gas (group)

2014

2000

1990

1970

Sulphate

-0.38

-0.42

-0.44

-0.36

BC

0.64

0.61

0.58

0.45

OC (incl. sec.)

-0.32

-0.31

-0.30

-0.27

Nitrate

-0.11

-0.11

-0.11

-0.05

Other gases

-0.60

-0.61

-0.61

-0.61

Total

-0.77

-0.84

-0.87

-0.84

Methodology for gap filling

If measurement data from a particular station are missing for a certain year, the global trend is derived from data available from other stations.

Data sources in latest figures

Uncertainties

Methodology uncertainty

Global average concentrations since approximately 1980 are determined by averaging measurements from several ground-station networks (SIO, NOAA/CMDL,ALE/GAGE/AGAGE), each network consisting of several stations distributed across the globe.

Absolute accuracies of global annual average concentrations are of the order of 1 % for CO2, CH4 and N2O, and CFCs; for HFCs, PFCs, and SF6, absolute accuracies can be up to 10 % -20 %. However, the year-to-year variations are much more accurate (IPCC, 2013).

Radiative forcing is calculated using parameterisations that relate the measured concentrations of greenhouse gases to radiative forcing. The overall uncertainty in radiative forcing calculations is shown in the tables below (given in 10 % and 90 % confidence ranges).

Best estimate values and 10 % and 90 % confidence ranges for total forcing (IPCC, 2013) are presented below:

Group

Forcers

Best estimate

10 %

90 %

Kyoto

CO2

1.82

1.63

2.01

CH4

0.49

0.44

0.54

N2O

0.18

0.15

0.21

HFC, PFCs, SF6

0.03

0.027

0.033

Montreal

Montreal F-gases

0.27

0.11

0.41

Non-protocol

Tropospheric O3

0.4

0.2

0.6

Sulfate aerosols

-0.40

-0.60

-0.20

Nitrate aerosols

-0.11

-0.19

0.03

Black carbon

0.64

0.22

1.02

Organic carbon

-0.32

-0.42

-0.22

Cloud Interaction

-0.55

-1.1

0.00

Land use

-0.15

-0.25

-0.05

Vapour

0.07

0.02

0.12

Mineral dust

-0.10

-0.30

0.10

Air contrails

0.05

0.00

0.10

Total

2.33

-0.06

4.70

Another way of showing the effect of uncertainties on the total greenhouse gas concentration is to use alternative values for the forcing of aerosols. New estimates of this forcing have been presented in previous years (IPCC, 2013). The cooling effect of aerosols is now estimated to be 0.35 W/m2 smaller (= more warming) than reported in previous years, mainly due to the greater warming potential of black carbon. When using these older values, the total greenhouse gas concentration in 2014 is estimated to be about 34 ppm lower than when using the more recent figures. Given a similar trend in time, this lower concentration level would imply an additional 10 years before the various critical concentration levels (Table 1) would be reached.

Overall, this shows the effect of a lower climate sensitivity (Rogelj et al., 2014) and higher heat uptake by the oceans (Doney et al., 2014). An important question here is whether this increased heat uptake will prove to be structural, over time.

Data sets uncertainty

Comparability of direct measurements is good. Although methods for calculating radiative forcing and CO2 equivalent are expected to be further improved, any update of these methods will be applied to the complete data set covering all years, so this will not affect the comparability of the indicator over time.

Rationale uncertainty

Atmospheric concentrations of greenhouse gases are a well-established indicator of changes in atmospheric composition, which causes changes in the global climate system.

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Long term work

Work specified here will require more than 1 year (from now) to be completed.